1. Field of the Invention
This invention relates generally to the field of circuit boards. In particular, this invention relates to substrates for circuit boards comprising a thermosetting composition, wherein the thermosetting composition comprises a polybutadiene/polyisoprene resin and a low molecular weight ethylene propylene rubber.
2. Brief Description of the Related Art
Circuit boards generally comprise an electrical substrate material laminated to a conductive metal layer, usually copper. Electrical substrate materials used in circuit boards are most commonly composites, comprising a polymeric matrix and an inorganic particulate and/or fibrous filler. Polybutadiene and polyisoprene resins and combinations thereof are particularly useful thermosetting compositions as described in commonly assigned U.S. Pat. No. 5,233,568 to Landi et al. and U.S. Pat. No. 5,571,609 to St. Lawrence et al., both of which are herein incorporated by reference in their entirety. U.S. Pat. No. 5,233,568 discloses a moldable thermosetting composition which is first formed into a shape, and then cured at a high temperature of greater than about 250.degree. C. U.S. Pat. No. 5,571,609 discloses a thermosetting resin comprising a polybutadiene or polyisoprene resin and an unsaturated butadiene- or isoprene-containing polymer in an amount of 25 to 50 volume percent (vol %); a woven glass fabric in an amount of 10 to 40 vol %; and a particulate filler in an amount of 5 to 60 vol %. This filled composite material leads to a prepreg which has very little tackiness and can therefore be easily handled by operators. The material in accordance with this patent has a low dissipation factor, a consistent, low dielectric constant, and a low coefficient of thermal expansion, and is therefor ideal for wireless communication and high speed, high frequency signal transmission applications.
Other polymeric matrices and substrates commonly used for circuit boards are disclosed in U.S. Pat. No. 5,264,065 to Kohm, which describes a base material for printed wiring boards where inert filler is used to control Z axis coefficient of thermal expansion in fiberglass reinforced thermoset resins. The Kohm patent discloses a range of 45-65 weight percent (wt %) fiberglass reinforcement and a range of 30 to 100 parts filler per 100 parts of the polymer. U.S. Pat. No. 4,997,702 to Grant et al. discloses a circuit laminate having an epoxy resin system which also includes inorganic fillers or fibers in the range of 20-70 wt % of the total composite. The fibers include both glass and polymeric fibers and the fillers include clay or mineral (e.g., silica) particulate fillers. U.S. Pat. No. 4,241,132 to Pratt et al. discloses an insulating board comprising a polymeric matrix such as polybutadiene and a filler consisting of polymeric filler (e.g., fibrous polypropylene). In all cases, the dielectric constant or dissipation factor of the resin matrix is matched to the fibrous reinforcement in order to obtain an isotropic composite.
European Patent No. 202 488 A2 discloses a polybutadiene based laminate wherein a high molecular weight, bromine-containing prepolymer is used to reduce tack and flammability of a 1,2-polybutadiene resin. Similarly, in Japanese Patent No. 04,258,658, a high molecular weight compound is added to a tacky polybutadiene resin to control tack. The compound utilized is a halogen-containing bismaleimide which provides flammability resistance as well as good copper bonding and heat resistance. There is no mention of the use of fillers and the resulting laminate has a relatively high dissipation factor. An article titled "A New Flame Retardant 1,2-Polybutadiene Laminate by N. Sawatri et al., IEEE Transactions on Electrical Insulation, Vol. EI-18, No. 2, April 1983 uses a high percentage of very high molecular polybutadiene and a low molecular weight, modified polybutadiene as a minor component, but there is no mention of the use of filler of any type.
The article entitled "1,2 Polybutadiene-High Performance Resins for the Electrical Industry", by R. E. Drake, ANTEC '84 pp. 730-733 (1984), generally discloses conventional polybutadiene resins for use in laminates and specifically discloses the use of reactive monomers which react with the polybutadiene. U.K. Patent Application 2 172 892A, generally discloses laminates composed of styrene-containing and thermoplastic copolymers with unsaturated double bonds and polybutadiene.
Application Ser. No. 09/132,869, filed Aug. 12, 1998, discloses polybutadiene or polyisoprene-based circuit board substrates comprising liquid ethylene propylene rubbers.
Such materials undergo a thermal cure at a high temperature (e.g., greater than about 250.degree. C.), without loss of copper bonding strength. Use of the ethylene propylene rubber and a styrene-butadiene block copolymer allows the materials to achieve the required high dielectric strength. However, such materials generally do not contain flame retardants due to the high temperature cure conditions.
While the above composites are well-suited for their intended purposes, the industry is constantly moving to the processing and use of circuit boards under ever more stringent environmental conditions. Properties such as dielectric strength should accordingly be well retained at the elevated temperatures which may be encountered during processing or use. Underwriters Laboratories, Inc. (UL) has developed protocols for characterizing the temperature limits that circuit boards and circuit board substrate materials can withstand during processing applications and under prolonged use. The UL Relative Thermal Index (RTI) identifies the temperature at which a material endures 100,000 hours of use before losing half of its initial dielectric break down strength or mechanical (tensile or flexural) strength. The RTI is generally thickness dependent, and so UL grants RTI's to circuit board materials at different thicknesses. Industry is furthermore requiring that circuit boards meet the UL Flammability test. Consequently, there is a perceived need in the art for circuit board substrates which have improved flammability resistance; improved time to the loss of initial dielectric and mechanical strength at a given temperature and at a given thickness; and improved retention of dielectric constant and mechanical strength at elevated temperatures, all while maintaining optimal electrical, chemical and processing characteristics as well as low cost.